Restore the document metadata dropped when the page was ported

[slachiewicz]

Unblocked by the release of plexus parent 26.

The APT source carried a title, an author and a date. Converting the page to Markdown
dropped them, so the generated page lost its <title> and its author meta. This puts
them back as YAML front matter, read from the deleted src/site/apt/index.apt:
author Hervé Boutemy, dated 2012-05-08.

It could not be done before now: parent 25 runs Spotless over **/*.md, and flexmark
rewrites the fence closing a front matter block, silently costing the page the very
metadata being restored. Parent 26 excludes **/src/site/markdown/**, so the first
commit here is the parent upgrade.

Verified after the bump — front matter survives the build and the page carries both:

<title>Plexus Compiler – Plexus Compiler</title>
<meta name="author" content="Hervé Boutemy" />

[slachiewicz]

Rewritten to drop the parent bump — parent POM updates belong in their own PR. The bump is
now #490, which this depends on: restoring the front matter before parent 26 is in place would
let Spotless eat it again.